1. Field of the Invention
The present invention relates in general to susceptors for semiconductor wafer processing. In particular, the present invention relates to an improved multipart susceptor for rapid thermal processing of semiconductor wafers.
2. Description of the Related Art
In the manufacture of semiconductor wafers, there are numerous process steps. One set of steps is referred to as epitaxial deposition, and generally consists of depositing a thin layer (between approximately 10 to less than one micron) of epitaxial silicon upon the wafer. This is achieved using specialized equipment and a chemical vapor deposition (CVD) process. The CVD process requires that the wafer be heated to very high temperatures, on the order of 1200.degree. C. (2000.degree. F.).
There has been a recent trend in this art to employ equipment which operates upon a single wafer, rather than a group of wafers. In this single wafer equipment the heating of the wafer to the CVD temperature is greatly accelerated, so that the wafer is typically taken from room temperature to the elevated temperature within approximately 30 seconds. This has been known as rapid thermal processing, or RTP. While the silicon wafer accepts this rapid temperature change well, the wafer must be held in position by a mechanical component known as a susceptor. To form a susceptor which will maintain its shape and still withstand the temperature change, the susceptors have been formed of graphite cores surrounded by a layer of silicon carbide.
While this has been acceptable for the rapid heating and CVD portion of the process, the deposition of the epitaxial silicon layer is not limited to the wafer. In other words, the silicon is also deposited upon the susceptor holding the wafer. To prevent excessive buildup of silicon, the susceptor is periodically cleaned by use of a hydrochloric acid vapor or similar etchants. This vapor removes the deposited silicon from the susceptor. However, it also removes small amounts of the silicon carbide layer, forming pin holes. This exposes the graphite core.
The manufacture of semiconductors requires very stringent standards for purity and cleanliness. While the silicon carbide coating on the susceptor is very inert and harmless to the process, graphite is not. As such, the formation of pinholes which expose the graphite core of the susceptor causes contamination of the wafer being produced. The number of pinholes increases with each cleaning of the susceptor, and as such so does the amount of contamination from the susceptor. At some point (typically about one to five thousand cycles) the amount of wafer contamination will be unacceptable and the susceptor must be replaced.